In January 1943 Tommy Flowers, a Royal Post Office engineer, was
sent to Bletchley Park to build an electronic machine to decipher
the Lorenz cipher, a complex encryption system used by Hitler and
the German High Command.

Over the next nine months, along with a few dozen technicians,
he built the first large, high-speed, electronic valve programmable
logic calculator. It measured 5 metres long, 3 metres deep and 2.5
metres high, and was assembled from telephone exchange parts.

Colossus could process 5,000 letters per second. It enabled the
Allies to eavesdrop on the German High Command before D-Day. The
deciphered high-grade signals intelligence was known as ULTRA and
it provided critical information on troop positions in Normandy
before the Allied invasion began on 6 June 1944.